headless: Support --use-gl in headless shell

headless/BUILD.gn

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//testing/test.gni")
 import("//tools/grit/grit_rule.gni")
 import("//tools/grit/repack.gni")
 
 config("headless_implementation") {
   defines = [ "HEADLESS_IMPLEMENTATION" ]
@@ skipping 258 matching lines @@
     ":pak",
     "//base",
     "//components/devtools_http_handler",
     "//content/public/app:both",
     "//content/public/browser",
     "//content/public/child",
     "//content/public/common",
     "//content/public/renderer",
     "//content/public/utility",
     "//net",
+    "//third_party/mesa:osmesa",
     "//ui/aura",
     "//ui/base",
     "//ui/compositor",
     "//ui/display",
     "//ui/ozone",
     "//url",
   ]
 
   configs += [ ":headless_implementation" ]
 }
@@ skipping 69 matching lines @@
   sources = [
     "app/headless_shell.cc",
     "app/headless_shell_switches.cc",
     "app/headless_shell_switches.h",
   ]
 
   deps = [
     "//headless:headless_lib",
   ]
 }